1. Field of the Invention
The present invention relates to an image enhancer to enhance an outline portion of an image.
2. Description of the Prior Art
For instance, in a video camera, VTR, and television receiver, an image enhancer is provided for a luminance signal system in order to enhance the outline portion of an image.
FIG. 1 is a system diagram of an example of a video camera. Reference numeral 1 denotes an image pickup section comprising a CCD and the like. An image pickup output signal from the image pickup section 1 is supplied to a processing circuit 3 through an amplifier 2. A luminance signal Y and color difference signals R-Y and B-Y of red and blue are derived from the processing circuit 3. The luminance signal Y is supplied to an image enhancer 4 and its outline is enhanced. Thereafter, it is supplied to an encoder 5. The color difference signals R-Y and B-Y are directly supplied to the NTSC encoder 5. A color video signal of, for instance, the NTSC system is obtained from the NTSC encoder 5 and is led to an output terminal 6.
Hitherto, the image enhancer 4 has been constructed as shown in FIG. 2.
Although the outline enhancement is executed in both of the horizontal and vertical directions of an image, since the principles of the enhancement are substantially the same, only the outline enhancement in the vertical direction will be described here.
That is, in FIG. 2, as shown in FIG. 3A, a consideration is made with respect to the case where the luminance signal Y having an outline with respect to the vertical direction is supplied to an input terminal 41.
The luminance signal Y transmitted through the input terminal 41 is supplied to an outline extracting circuit 42 and the outline portion in the vertical direction of the image is extracted. As the outline extracting circuit 42 in the vertical direction, there is frequently used a construction which uses on delay circuit of one horizontal period and obtains a primary differential outline signal as shown in FIG. 3B or a construction which uses two delay circuits of one horizontal period and obtains a secondary differential outline signal as shown in FIG. 3E.
As shown in FIGS. 3B and 3E, in the portions other than the outline, an outline signal S.sub.1 obtained from the outline extracting circuit 42 includes micro amplitude noises of the circuit system. The noises increase when the gain is raised. Therefore, the outline signal S.sub.1 is supplied to a noise slicing circuit 43 and the micro amplitude noises are eliminated. As shown in FIGS. 3C and 3F, an outline signal S.sub.2 from which the micro amplitude noises are substantially eliminated is obtained.
The outline signal S.sub.2 from the noise slicing circuit 43 is supplied to an adding circuit 45 through a band limiting low-pass filter 44 for suppressing bands other than an objective frequency band whose outline is to be enhanced.
The filter 44 is provided to suppress the generation of a cross color and to suppress the appearance of the horizontal component in the outline in an oblique fringe. Its cut-off frequency is selected to 2 to 3 MHz.
The adding circuit 45 adds the luminance signal Y which is obtained from the input terminal 41 through a timing adjusting delay circuit 46 and the outline signal S.sub.2 from the filter 44. A signal YA which was outline enhanced as shown in FIGS. 3D and 3G is derived and led to an output terminal 47.
A delay amount DL of the delay circuit 46 is set in consideration of the delay in the outline extracting circuit 42. For instance, in the case of extracting the outline by using one delay circuit of one horizontal period, the delay amount DL is set to zero. In the case or extracting the outline by using two delay circuits of one horizontal period, the delay amount DL is set to a value of one horizontal period.
In many cases, the outline extracting circuit 42 and noise slicing circuit 43 at the front stage of the filter 44 are constructed as a single IC. This is because the filter 44 is difficult to construct as an IC and it is arranged after the noise slicing circuit 43, so that the input and output terminals of the IC can be reduced.
However, in the case of the conventional image enhancer with the construction as mentioned above, since the filter 44 is arranged as the last stage of the outline signal forming system, the outline signal S.sub.2 which is added to the luminance signal Y becomes unsharp as shown in FIG. 4 because of the band limitation of the filter 44 and the waveform characteristic deteriorates.
On the other hand the noise slicing circuit 43 cannot eliminate all of the noises and cannot eliminate the noises exceeding the slicing range. Among the noises which cannot be eliminated by the noise slicing circuit 43, are the noises whose frequencies ar sufficiently higher than the cut-off frequency of the filter 44. However, there is a drawback that the noises near the cut-off frequency and the noises of the frequencies lower than it remain.